How does a radio interferometer combine signals from separate antennas?

A radio interferometer combines signals from separate antennas by utilizing the principles of interference to synthesize a large effective aperture. Because radio waves reach spaced antennas at different times, the system must compensate for this path length difference to allow the waves to interfere constructively or destructively.12

This process is achieved through several key mechanisms:

• Variable Delay: A variable-delay mechanism is used to compensate for the time difference in signal arrival at each antenna, allowing the waves to be combined effectively.2
• Phase Interaction: As Earth rotates, the path difference between the antennas and the radio source changes, causing the signals to alternate between constructive interference and destructive interference.1
• Fourier Synthesis: Each pair of antennas measures a "Fourier component" of the source's brightness distribution. By using multiple antenna spacings and Earth's rotation, astronomers collect enough data to reconstruct high-resolution images using the fast Fourier transform (FFT).1

This technique, known as aperture synthesis, allows radio telescopes to achieve angular resolution far superior to what would be possible with a single, smaller antenna.12